Use <link href="/home/marc/GitProjects/markdown-css/markdown.css" rel="stylesheet"></link> on top of the markdown document
<link href="/home/marc/GitProjects/markdown-css/markdown.css" rel="stylesheet"></link>
Conversion with pandoc -t html5 --metadata title="<title>" <file>.md -o <file>.pdf
pandoc -t html5 --metadata title="<title>" <file>.md -o <file>.pdf
To test the html output: pandoc <file>.md -o <file>.html
pandoc <file>.md -o <file>.html